A User Equipment (UE), sometimes called mobile station or terminal, in connected mode can be tracked either on cell level, which means that the Radio Network Control (RNC) keeps track of in which cell the UE is located, or on Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) Registration Area (URA) level. A URA covers a number of cells. In the states URA_PCH and CELL_PCH (PCH=Physical Channel) there are no possibilities to send user data. The RNC can only reach the UE by means of UTRAN paging. In CELL_FACH (FACH=Forward Access Channel) state the UE has been assigned a common Random Access Channel (RACH) that can be used for user data transmission in Uplink (UL).
However, special channel access procedures such as power ramping, collision avoidance and contention resolution must be used before uplink data transmission can begin, as multiple terminals share the channel. The channel assigned for downlink data (Forward Access Channel, FACH) is also shared by multiple terminals, but access to it is scheduled by the RNC. In the state CELL_DCH (DCH=Dedicated Channel) a dedicated or shared physical channel has been assigned to the UE.
Previous to Rel-7 (Release 7 of 3GPP TS 25.214 “Physical layer procedures” (FDD)), a UE which was in state CELL_FACH had to switch to state CELL_DCH in case the UE wanted to read High Speed-Shared Control Channel/High Speed-Downlink Shared Channel (HS-SCCH/HS-DSCH). This implied that several control messages had to be exchanged between the UE and the Base Station also called Node-B or evolved Node-B (eNode-B) in Long Term Evolution (LTE), and between Node-B and RNC; therefore delays were considerably large. Release 7 introduced a feature in which Node-B could address a UE using the HS-SCCH/HS-DSCH while the UE's state was CELL_FACH. As a result, delays were reduced and performance improved.
However, UE may address the Node-B when the UE has data to be transmitted. If the UE is in state CELL_FACH, the UE needs to access the network through a Random Access Channel (RACH) procedure, synchronize in both Uplink (UL) and Downlink (DL), and move to state CELL_DCH, in order for the UE to start transmitting data. This procedure also requires some exchange of control messages between Node-B and RNC.
One disadvantage with the RACH procedure is emphasized when the UE needs to transmit bursty traffic, i.e. relatively small amount of data per non-defined time period, such as web browsing traffic.
In this case a UE whose state is CELL_FACH needs to switch to CELL_DCH state before it can send anything but very small amount of data. The procedure of switching may take several hundreds of milliseconds. This is highly inconvenient when UE has relatively small amount of data to be sent, but still more than is feasible to transmit on current RACH, as the experienced throughput would be very low due to the large set-up time.
After sending the data, UE will eventually transit again to state CELL_FACH after the inactivity period has expired. When the UE needs to send data once more, it will have to go through the same procedure again.